1. Field of the Invention
This invention relates to cameras for daylight and flash photography, and more particularly to a triggering arrangement for controlling the operation of a flash unit associated with a camera in conformity with the operation of the camera shutter to establish a uniform flash exposure over the entire area of the film at shutter speeds wherein the running-down movement of the trailing shutter curtain is factored into the flash exposure determination.
2. Description of the Prior Art
The conventional type of trigger devices are so designed that a flash unit associated with a camera operates with only one of the paired shutter blades or curtains of the camera, namely, the leading shutter curtain, in such a manner that the firing of the flash lamp is initiated at a point in time when the leading shutter curtain arrives at the opposite terminal end position as it runs down in front of and across the film aperture. With this trigger device, however, when the shutter control mechanism is set to a fast shutter speed position where the ends of the leading and trailing curtains are spaced from each other by a distance smaller than the longitudinal length of the film aperture, in other words, when the shutter curtains allow only a narrow band of the image-forming light to travel across the film and to exposure the film, the magnitude of the exposure is non-uniform in different regions of the film. For better understanding of this phenomenon, reference may be had to FIGS. 1A and 1B of the attached drawings.
FIG. 1A shows the variation with time of the position of either of the leading and trailing shutter curtains relative to the other as they travel across the focal plane of the camera to make a daylight exposure in connection with the duration of the firing of a flash lamp for which an exposure under combined daylight and flash lighting, or complex lighting conditions, is made. In this figure, the abscissa represents time, and the ordinate represents the longitudinal length of an exposure aperture at the focal plane for curves designated by the characters, a, b.sub.0 and b.sub.1 through b.sub.7, representing various exposure time intervals. The ordinate also represents the magnitude of the flash output energy for a curve A available from an electronic flash unit and for curves B and C from bulb type flash units. When the camera shutter is released to make an exposure, the rear end of the leading shutter curtain traverses the camera focal plane as shown by curve a. That is, the rear end of the leading shutter curtain traverses one end of the effective area of the image at a point in time coincident with the origin of the coordinate axes and reaches the opposite end of the image at a point in time designated by t.sub.1. At the time t.sub.1 the entire film gate is open and the exposure of the film over the entire effective area of the image or through the film gate is uniformly established as the trailing shutter curtain is retained in the unblocked position.
According to the prior art, therefore, the trigger device is so constructed to initiate the firing of the flash lamp at this time t.sub.1. As the electronic type flash lamp unit operates in a time period between times t.sub.1 and t.sub.2, the front end of the trailing curtain begins blocking the film gate as shown by one of curves b.sub.1 through b.sub.4 with times t.sub.2 through t.sub.5 respectively, subsequent to the operation of the flash unit whereby a uniform flash exposure is effected over the entire area of the image. Now assuming that the trailing curtain is caused to block the film gate from the image-forming light in such a manner as shown by curve b.sub.0 beginning with time t.sub.0, an open slit is formed in the shutter, this shutter slit being moved across the film so that when the flash lamp is triggered at the time t.sub.1, the exposure is different in two portions of the image as shown in FIG. 1B, wherein the hatched portion of the image is blocked by the trailing curtain and is therefore taken in the ambient lighting condition alone, while the other portion is taken under the complex lighting condition. In the case of the latter type of flash units such as those employing M grade or FP grade flash bulbs in which the firing duration is relatively long as shown by curves B and C of FIG. 1A, the blocking of the film gate must be initiated at a point in time, for example, 30 to 40 milliseconds, after the initiation of establishment of the entire unblocking of the film gate as indicated by curves b.sub.5 to b.sub.7. If this precaution is not taken, only part of the flash flash light energy produced by the flash bulb would be used, although the film would be uniformly exposed over the entire area of the image.
With the foregoing in mind, it has been common in the prior art to adjust the shutter speed to a certain value, for example, usually 1/60 second each time an exposure is made in the flash mode. This adjustment is usually carried out by manipulating the shutter dial of the camera, and therefore is liable to be overlooked, particularly when a flash exposure is to be made between daylight exposures. Moreover, there is a possibility of accidentally setting the shutter dial to a shutter speed faster than 1/60 second at one or more times in the course of a series of flash exposures. Such an accident is undetectable as long as the photographer is forced to be aware of the subject lighting condition derived from the flash unit, because no change occurs in the subject lighting condition by the accident. To ignore such accidental setting of the shutter dial is to limit the percentage of photographs taken by flash illumination which will be found acceptable.